1. Field of the Invention
The invention relates to cellulose derivatives having gel-like properties and a process for their preparation.
2. Brief Description of the Prior Art
Cellulose derivatives, owing to their excellent properties and physiological safety, are used widely, for example as thickeners, adhesives, binders and dispersants, water-retention agents, protective colloids, stabilizers and suspension, emulsifying and film-forming agents. Conventional commercially available cellulose derivatives which are soluble in water, for example methylhydroxyethyl cellulose, methylhydroxypropyl cellulose and hydroxyethyl cellulose, exhibit a characteristic rheological profile which may be described on the basis of material functions of the aqueous solution of the cellulose derivative. Aqueous solution in this case denotes a system which comprises water, cellulose derivative and, where there is present, salts and accompanying substances of the cellulose derivative and the water used, for example tap water. Material functions discussed are usually the viscosity η as a function of shear rate {dot over (γ)} for describing the flow properties, and the storage modulus G′ and the loss modulus G″ in each case as a function of the angular frequency ω for describing linear viscoelastic properties. The symbols used here follow the recommendations of the publication: C. L. Sieglaff: “Proposed Nomenclature for Steady Shear Flow and Linear Viscoelastic Behavior”, Transactions of the Society of Rheology 20:2 (1976) 311-317.
In the case of viscosity, generally the complete function η({dot over (γ)}) is not given, but a representative viscosity value which is determined under defined conditions with respect to concentration of the cellulose derivative in the aqueous solution, the temperature and the shear rate and also the measuring instrument used and the instrument settings. This procedure is well known to those skilled in the art. It is also generally known that in most cases the viscosity of the aqueous solution of a cellulose derivative decreases with increasing shear rate; the aqueous solutions thus exhibit pseudoplastic flow behaviour.
The linear viscoelastic properties are determined by measurements in an oscillating shear flow at small amplitude and with variable angular frequency. The values for G′ and G″ are determined to a great extent here by the concentration of the cellulose derivatives in the aqueous solution and the magnitude of the representative viscosity value. Therefore, hereinafter, only the relative course of G′ and G″ with increasing angular frequency ω is considered. At a concentration of 1.5 to 2 parts by weight of cellulose derivative per 100 parts by weight of aqueous solution and a temperature of approximately 20° C., the behaviour of G′ and G″ for the cellulose derivatives of the prior art is such that at a low angular frequency ω, the storage modulus G′ is less than the loss modulus G″, but with increasing angular frequency G′ increases more greatly than G″. On occasions, G′, above a certain angular frequency, finally becomes greater than G″, and the solution at high values of angular frequency thus predominantly reacts elastically.
For conventional cellulose derivatives, in aqueous solution the dependence on the angular frequency is therefore considerably greater for G′ than for G″. In particular, the linear viscoelastic material functions, storage modulus G′ and loss modulus G″, in the range of angular frequency ω of 0.1 s−1 to 1 s−1 depend on the angular frequency in such a manner that the exponents n and m in the relationships:                (1) G′∝ωn (storage modulus is proportional to the angular frequency to the power of n)and        (2) G″∝ωm (or (loss modulus is proportional to the angular frequency to the power of m)differ markedly, wherein the ratio of n to m is greater than 1.20.        
Besides the effect of increasing the viscosity, requirements for an optimum setting of the rheological properties of aqueous systems, by the use of cellulose derivatives, can include gel-like properties. Here, for example, methylhydroxyethyl cellulose or methylhydroxypropyl cellulose, which exhibit a thermal flocculation point in water, offer the opportunity of forming gels in a temperature-dependent manner. See N. Sarkar: “Kinetics of thermal gelation of methylcellulose and hydroxypropylmethylcellulose in aqueous solutions”, Carbohydrate Polymers 26 (1995) 195-203. The dependence on the angular frequency is no longer markedly greater for G′ in gel-like systems than for G″.
Achieving gel-like properties by utilizing the thermal flocculation point with setting-defined temperatures involves a marked restriction on the use of cellulose derivatives from two aspects: firstly, it is necessary to, set, with a certain effort, the temperatures suitable for reaching the gel-like properties. Secondly, the selection of the cellulose derivatives is restricted to the products which have a flocculation point in the desired temperature range.
Achieving gel-like properties by partial or-complete replacement of cellulose derivatives by other hydrocolloids which impart gel-like properties is frequently undesirable, since, as a result, certain properties of the cellulose derivatives, for example good water retention, are no longer completely available. Also, such hydrocolloids are generally not based on renewable raw materials or are not biodegradable.
There is therefore a requirement for cellulose derivatives which have gel-like rheological properties in aqueous solution without the addition of other substances or a particular temperature profile being required.
Thus GB 514,917 already describes a process for preparing water-soluble cellulose ethers which are crosslinked with a bifunctional reagent. The purpose of GB 514,917 was to prepare cellulose ethers which have an unusually high viscosity in water. Preferably, the products display a viscosity increase of 400%.
U.S. Pat. No. 4,321,367 also describes a process for preparing crosslinked cellulose ethers, again with the purpose of providing products of increased viscosity in aqueous solution. Preferably, the viscosity of a 2% strength by weight solution is increased by at least 50%; in the most-preferred variant, the viscosity of a 2% strength by weight solution is increased by at least 100%. A surfactant is added, inter alia, as additive to the reaction mixture in order to achieve the distribution of the reactants.
The processes described in these publications are in part multistep, require additional additives such as surfactants, and give only low yields with respect to the crosslinking reagent. The viscosity of the cellulose ethers, compared with the uncrosslinked cellulose ethers is increased extremely greatly, as a result of which the experimental results of such processes are virtually not reproducible.
For these reasons, no commercially usable products have resulted from this group of products.